add Async::Promise

lib/async/promise.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2025, by Patrik Wenger.
+
+require_relative "variable"
+
+module Async
+
+	# Similar to a Variable, in that it allows a task to wait for a value to resolve, but also supports rejection given
+	# an error. The given error is raised as an exception in the waiting task.
+	class Promise < Variable
+		def initialize(...)
+			super
+
+			@error = nil
+		end
+
+		def reject(error = RuntimeError.new('promise rejected'), value = nil)
+			@error = error
+			resolve value
+		end
+
+		def rejected?
+			resolved? && !!@error
+		end
+
+		# @returns [Boolean] Whether the value has been resolved.
+		# @raises [Exception] The error with which this Variable was rejected
+		def wait
+			value = super
+			raise @error if @error
+			return value
+		end
+	end
+end

readme.md

@@ -6,7 +6,7 @@ Async is a composable asynchronous I/O framework for Ruby based on [io-event](ht
 > [tus-ruby-server](https://github.com/janko/tus-ruby-server) – would really benefit from non-blocking I/O. It's really
 > beautifully designed." *– [janko](https://github.com/janko)*
 
-[![Development Status](https://github.com/socketry/async/workflows/Test/badge.svg)](https://github.com/socketry/async/actions?workflow=Test)
+[![Development Status](https://github.com/socketry/async/workflows/Test/badge.svg)](https://github.com/socketry/async/actions?workflow=Test) [<img src="https://api.gitsponsors.com/api/badge/img?id=87380483" height="20">](https://api.gitsponsors.com/api/badge/link?p=U4gCxvzG7eUksiJSe0MSlN/IQp/rOEmM4wKYQwPrkiFn78vaBdtOdFQLsJu4pzyegaVzeek/1axVEWxd6fq1Ww==)
 
 ## Features
 

test/async/promise.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2025, by Patrik Wenger.
+
+require "sus/fixtures/async"
+require "async/promise"
+
+PromiseContext = Sus::Shared("a promise") do
+	let(:promise) {Async::Promise.new}
+
+	it "can resolve the value" do
+		promise.resolve(value)
+		expect(promise).to be(:resolved?)
+	end
+
+	it "can wait for the value to be resolved" do
+		Async do
+			expect(promise.wait).to be == value
+		end
+
+		promise.resolve(value)
+	end
+
+	it "can wait for the value to be resolved using setter" do
+		Async do
+			expect(promise.wait).to be == value
+		end
+
+		promise.value = value
+	end
+
+	it "can't resolve it a 2nd time" do
+		promise.resolve(value)
+		expect do
+			promise.resolve(value)
+		end.to raise_exception(FrozenError)
+	end
+
+	it "can reject with an exception" do
+		promise.reject RuntimeError.new("boom")
+		expect(promise).to be(:resolved?)
+		expect(promise).to be(:rejected?)
+	end
+
+	it "can wait for a rejection with an error" do
+		Async do
+			expect do
+				promise.wait
+			end.to raise_exception(RuntimeError)
+		end
+
+		promise.reject(RuntimeError.new)
+	end
+end
+
+include Sus::Fixtures::Async::ReactorContext
+
+describe true do
+	let(:value) {subject}
+	it_behaves_like PromiseContext
+end
+
+describe false do
+	let(:value) {subject}
+	it_behaves_like PromiseContext
+end
+
+describe nil do
+	let(:value) {subject}
+	it_behaves_like PromiseContext
+end
+
+describe Object do
+	let(:value) {subject.new}
+	it_behaves_like PromiseContext
+end

test/async/variable.rb

@@ -21,6 +21,14 @@
 
 		variable.resolve(value)
 	end
+
+	it "can wait for the value to be resolved using setter" do
+		Async do
+			expect(variable.wait).to be == value
+		end
+
+		variable.value = value
+	end
 
 	it "can't resolve it a 2nd time" do
 		variable.resolve(value)